1. Field of the Invention
The present invention relates generally to radiation detectors and more particularly to semiconductor photodetectors.
2. Description of the Related Art
Optically immersed radiation detectors have a detector element that is placed in optical contact with a lens that preferably has a high index of refraction. The term of art used to describe the detector element optically contacting the lens is that the detector element is "immersed to" the lens. Elimination of intervening materials, e.g., air, that have a lower refraction index permits the lens to image a greater solid angle of incident radiation onto the detector element. In immersed radiation detectors, the lens acts as the final optical unit that collects radiation for the detector element.
The performance gain of optically immersed detectors was analyzed and reported in R. Clark Jones, "Immersed Radiation Detectors", Applied Optics, Vol. 1, No. 5, September 1962. This paper investigated the gain due to immersion when the detection system is in the presence of externally and internally generated noise.
U.S. Pat. No. 2,964,636 was directed to optically immersed photoconductive cells that were formed by deposition of a lead sulfide or lead selenide photosensitive film on one surface of an imaging lens, e.g., on the flat surface of a plano-convex lens.
Experiments with the immersion of photodetectors with hemispherical germanium lenses were conducted and reported by Slawek, Joseph E. Jr., et al, "Letter to the Editor", Infrared Physics, Vol. 15, 1975, pp. 339-340. These experiments were specifically directed to photoconductive detectors formed with the compound semiconductor HgCdTe (mercury-cadmium-telluride) and demonstrated a loss in radiation responsivity when bonding adhesives were introduced between the photoconductive detector and the lens.